A diminished visual acuity or total loss of vision may result from a number of eye diseases or disorders caused by dysfunction of tissues or structures in the anterior region of the eye and/or posterior region of the eye. The eye is divided anatomically into an anterior and posterior segment. The anterior segment includes the cornea, anterior chamber, iris and ciliary body (anterior choroid), posterior chamber and crystalline lens. The posterior segment includes the retina with optic nerve, choroid (posterior choroid) and vitreous. The posterior portion of the eyeball supports the retina, choroid and associated tissues.
Examples of eye disorders resulting from the pathologic conditions of structures in the anterior segment of the eye are dry eye syndrome, keratitis or corneal dystrophy, cataracts, and glaucoma. Disease or disorders of the posterior segment of the eye in general are retinal or choroidal vascular diseases or hereditary diseases such as Leber Congenital Amaurosis. Age related macular degeneration (AMD) is one of the specific diseases associated with the posterior portion of the eyeball and is the leading cause of blindness among older people. AMD results in damage to the macula, a small circular area in the center of the retina. Because the macula is the area which enables one to discern small details and to read or drive, its deterioration may bring about diminished visual acuity and even blindness. The retina contains two forms of light receiving cells, rods and cones, that change light into electrical signals. The brain then converts these signals into the images. The macula is rich in cone cells, which provides central vision. People with AMD suffer deterioration of central vision but usually retain peripheral sight.
There are several types of AMD. The “dry” (non-exudative) type accounts for about 90% of AMD cases. The “wet” (exudative) form afflicts only about 10% of AMD patients. However, the wet form is a more serious disease than the dry form and is responsible for about 90% of the instances of profound visual loss resulting from the disease. Wet AMD often starts abruptly with the development of tiny, abnormal, leaky blood vessels termed CNVs (chorodial new vessels), directly under the macula. In most patients, this leads to scarring and severe central vision loss, including distortion, blind spots, and functional blindness.
Signs of AMD such as drusen, which are abnormal yellow deposits under the retina, can be present even in patient with normal vision. Drusen look like specks of yellowish material under the retina. They are deposits of extracellular material that accumulate between retinal pigment epithelium (RPE) and Bruch's Membrane. The RPE is a specialized cell layer that ingests used-up outer tips of the rod and cone cells and provides them with essential nutrients (e.g., vitamin A derivatives). Brach's membrane is a noncellular structure (composed mostly of collagen) that separates the RPE from the choroidal circulation below. The choroidal circulation provides blood supply to the rods, cones and RPE cells. A few small drusen normally form in the human eye, usually after age 40. AMD, in contrast, is almost always associated with a build-up of additional drusen. Drusen occur in two forms. Hard drusen are small, solid deposits that apparently do no harm when present in small numbers. Soft drusen are larger and may have indistinct borders. As soft drusen build up between the RPE and Bruch's membrane, they lift up the RPE and force the two layers apart.
Drusen develop long before the abnormal vessels of wet AMD. Three characteristics of soft drusen are risk factors for developing CNV: The presence of five or more drusen deposits; drusen size greater than 63 micrometers (about the thickness of a human hair); and the clumping of the drusen deposits. Some evidence suggests soft drusen are instrumental in the spread of abnormal vessels, but whether they stimulate vessel growth (angiogenesis) or simply provide space for them by lifting up the RPE remains unclear.
Two networks of blood vessels nourish the retina, one located on the retinal surface and the other located deep in the retina, external to Bruch's membrane. The abnormal vessels of AMD originate in the lower network of vessels, called the choroidal circulation. These vessels make their way through Bruch's membrane and spread out under the RPE. Blood and fluids leak from them and cause the photoreceptor cells to degenerate and the macula to detach from the cells under it.
Slightly blurred or distorted vision is the most common early symptom of AMD. Visual loss with dry AMD usually progresses slowly while visual loss with wet AMD proceeds more rapidly and may occur over days or weeks. Patients who have wet AMD in one eye are at increased risk of developing CNVs in the other eye. The magnitude of the risk varies, depending on the appearance of the second eye. The risk is greater in eyes with numerous large drusen, with abnormal pigment changes in the macula, and in patients with a history of high blood pressure.
AMD is now the leading cause of legal blindness in the western world. Reactions that go on in the RPE lead to oxidative products that in turn lead to cell death and neovascularization. This excess metabolism leads to the formation of drusen under the RPE.
Other eye diseases also affect photoreceptor function in the eye. Retinitis Pigmentosa represents disease caused by defects in many different genes. They all have a final common pathway of night blindness and peripheral vision loss that can lead to narrowing of the visual field and eventual loss of all vision in many patients. The rod photoreceptors are usually primarily affected and most of the gene defects leading to the disease occur in genes that are expressed predominantly or only in the rod cells.
One autosomal dominant form of Retinitis Pigmentosa comprises an amino acid substitution in opsin, a proline to histidine substitution at amino acid 23. This defect compromises 10-20% of all Retinitis Pigmentosa cases. This abnormal opsin protein forms a protein aggregate that eventually leads to cell death.
Leber Congenital Amaurosis is a very rare childhood condition that affects children from birth or shortly there after. It affects both rods and cones. There are a few different gene defects that have been associated with the disease. These include the genes encoding the RP65 and LRAT proteins. Both result in a person's inability to make 11-cis-retinal in adequate quantities. In the RP65 defective individuals, retinyl esters build up in the RPE. LRAT-defective individuals are unable to make esters and subsequently secrete any excess retinoids.
Retinitis Punctata Albesciens is another form of Retinitis Pigmentosa that exhibits a shortage of 11-cis-retinal in the rods. Aging also leads to the decrease in night vision and loss of contrast sensitivity due to a shorting of 11-cis-retinal. Excess unbound opsin is believed to randomly excite the visual transduction system. This can create noise in the system, and thus more light and more contrast is necessary to see well.
Congenital Stationary Night Blindness (CSNB) and Fundus Albipunctatus are a group of diseases that are manifested as night blindness, but there is not a progressive loss of vision as in the Retinitis Pigmentosa. Some forms of CSNB are due to a delay in the recycling of 11-cis-retinal. Fundus Albipunctatus until recently was thought to be a special case of CSNB where the retinal appearance is abnormal with hundreds of small white dots appearing in the retina. It has been shown recently that this is also a progressive disease although much slower than Retinitis Pigmentosa. It is caused by a gene defect that leads to a delay in the cycling of 11-cis-retinal.
Currently, there are few treatments for retinoid deficiency. One treatment, a combination of antioxidant vitamins and zinc, produces only a small restorative effect. Thus, there is a need for compositions and methods of restoring or stabilizing photoreceptor function and ameliorating the effects of deficient levels of endogenous retinoids.